Silica aerogels differs from general-purpose heat-insulation materials such as urethane forms (polyurethane; PU), expanded polystyrene (EPS), and vacuum insulation panels (VIPs), in that their heat-insulation performance hardly changes across the ages. Furthermore, since silica aerogels have a heat resistance of 400° C. or higher, they have attracted a great deal of attention as next-generation heat-insulating materials.
Silica aerogels are superior to existing heat-insulation materials in terms of resistance to age-related deteriorations, and heat resistance, and have excellent, heat conductivity around 15 mW/mK. However, in silica aerogels, network structures in which a string of silica particles on the scale of several tens of nanometers are beaded by point contact, and therefore, silica aerogels do not have sufficient mechanical strength. In order to cope with such fragility, there have been studies in which improvement of mechanical strength of silica aerogels is attempted based on combination with fibers, unwoven fabrics, resins, etc.
Heat-insulation materials made of silica aerogels easily produce radiation heat on the surfaces of the silica particles having a high radiation factor (about 0.95), under a high-temperature environment, e.g., above 100° C.
Consequently, influences of the radiation heat transmission become dominant over heat insulation properties obtained by pores possessed by the silica particles, and thus, there are tendencies that silica aerogels lose sufficient heat-insulation properties (apparent heat conductivities of the heat-insulation materials become larger). That is, silica aerogels absorb infrared rays from the outside, and then, release them to the outside. Accordingly, the silica aerogels transmit heat therethrough.
As one conventional art that addresses the above problem, there is a publication of Japanese Patent No. 3,854,645. A substrate that is obtained by attaching, onto unwove fabrics or a mat-shaped fiber material, silica aerogels having silica frameworks. At least one part of a surface of the substrate is seated with a coating layer including alumina (having low a radiation factor). This coating layer suppress the influences of the radiation heat transmission.